1. Field of the Invention
This invention relates generally to endless polyurethane belts and, more particularly, to improved belts resulting from enhanced elastomer composition and formation. Specifically, the present invention relates to an improved endless belt in the form of power transmission belts, V-belts, micro-ribbed belts and the like, having enhanced temperature resistance properties while undergoing dynamic load during operation.
2. Description of the Prior Art
It has been recognized that endless belts, such as power transmission belts, V-belts, micro-V or micro-ribbed belts, and the like, constructed from polyurethane elastomers offer a number of significant advantages over conventional rubber belting. Among these advantages are that an elastomeric belt is less susceptible to flex fatigue, can be driven around smaller sheaves, and demonstrates improved load life. These belts may be vacuum spin cast in a single operation, injection molded, or batch casted as compared to the numerous steps required to build conventional rubber tooth-type belts. However, many of the castable timing belts, and particularly those constructed of urethane, are often noisier than a comparable rubber belt due in part to a difference in the coefficient of friction of the material. Urethane belts have a higher modulus and are generally more aggressive as they enter and leave a sprocket or sheave and build up considerable heat at the interfaces. This heat buildup reduces the efficiency of the belt, and the higher operating temperatures can change the modulus and reduce belt life considerably by lowering the tear strength of the teeth or by attacking the bond between the elastomeric body and the tensile cord embedded therein.
Examples of power transmission belts, V-belts and micro-ribbed belts are disclosed in U.S. Pat. Nos. 3,138,962, 3,200,180, 4,330,287 and 4,332,576. Examples of formation of such belts are readily disclosed in U.S. Pat. No. 3,200,180 as indicated above and U.S. Pat. Nos. 3,772,928 and 4,066,732. These patent references are merely examples of the types of belts and state of the art of formation thereof.
One solution to the noise and heat buildup problem in such belts has been to reduce the coefficient of friction of the sheath engaging surface of the belt by isolating or removing as much of the elastomer as possible from near the surface of the belt which comes in contact with the sprocket teeth or flanges. Such an approach is taken in U.S. Pat. No. 3,772,929. Another way of dealing with the noise and heat degeneration problem is disclosed in U.S. Pat. No. 3,964,328. In this particular patent reference, a layer of elastomer impervious material is utilized during the casting operation and bonded to one side of a wear-resistant fabric.
The references provided above deal primarily with spin cast and injection molded polyurethane-based elastomers. Such polyurethane-based elastomers are often prepared by reacting a relatively high equivalent weight active hydrogen-containing material such as a polyol, and a relatively low equivalent weight active hydrogen-containing material, such as a chain extender, with a polyisocyanate. In preparing the elastomer, the reactive components and any catalyst or other optional additives are generally blended and reacted together and then transferred to a mold of suitable shape where the formulation is cured. In typical injection molding, the mixed material is reacted and heated and then injected into a cold mold to solidify and cure the product. Any tensile members for belt reinforcement are previously placed in the mold. It is typical practice to cure the elastomer in the mold until it is capable of maintaining the molded shape, and then demolding the elastomer and post-curing it until the polymerization is complete. In this manner, the mold may be used more often thereby permitting higher production rates.
Since it is usually desirable to produce as many molded parts, and therefore as many belts, as possible in a given period of time, it is important that the residence time in the mold be as short as possible. Accordingly, it is desirable that the elastomer formulation cure relatively rapidly in the mold to a state which the elastomer can be demolded and postcured. In batch processing, however, it is necessary that the formulation not cure too quickly since some time is required to blend the batch components of the formulation and then transfer the blend to the mold. Once the elastomer sheath has been demolded and postcured, it is then cut into belts.
In addition to batch processing and standard thermoplastic injection molding, Reaction Injection Molding (RIM) is a technique for the rapid mixing, reacting and molding of large, fast curing urethane parts. While RIM polyurethane parts have traditionally been used in a variety of exterior body applications on automobiles where their light weight contributes to energy conservation, RIM polyurethane parts have not typically been used for dynamic application such as in the formation of belts. RIM parts are generally made by rapidly mixing active hydrogen containing materials with polyisocyanate and simultaneously injecting the mixture into a mold where reaction proceeds These active hydrogen containing materials typically include a high molecular weight polyhydric polyether and/or a low molecular weight active hydrogen containing compound, for example, a chain extender Moreover, RIM parts for automobile applications typically are reacted very quickly and demold in 1-2 minutes. After reaction and demolding, the parts may be subjected to an additional curing step by placing them at an ambient temperature or about 250.degree. F. or greater for 4-24 hours. Unfortunately, the extreme rapid reaction time may causes a loss of control over the morphological structure.
Typical RIM elastomers and their preparation include U.S. Pat. Nos. 4,806,615, 4,742,090, 4,404,353, 4,732,919, 4,530,941 and 4,607,090. Typical of accepted RIM practice is to place all components except for the isocyanate in one vessel (B-side) and the isocyanate in another vessel (A-side) prior to reaction, and then admixing these A and B side components together in a mold. U.S. Pat. No. 4,297,444 discloses a modification to this traditional procedure. In this modification, the reacting of a portion of the high molecular weight polyether with a portion of the isocyanate is performed, while the chain extender and remaining polyether are admixed together along with the prepolymer in a RIM process to react the components to form a RIM polyurethane elastomer.
As indicated previously, RIM elastomers have been readily utilized as automobile fascia and other components thereof, such as fenders, steering wheels, dash boards, and various other structural and flexible components. The significant advantage in the RIM processing technique is that admixing, reaction and molding injection all take place simultaneously to reduce the amount of residence time in the mold. Thus, RIM elastomers have found wide acceptance in a variety of consumer and industrial applications.
However, as indicated above, certain product applications necessitating the use of an endless belt require that the belt be subjected to external, dynamic loading as opposed to static and/or non-loaded applications. Moreover, in certain applications such as automobile timing and power transmission belts, V-belts, micro-ribbed belts, and the like, such belts are subjected to both high and low temperature extremes in dynamic loading conditions. In such situations, polyurethane elastomer belts have to date been unacceptable for long term usage due to their tendency to yield and/or crack under dynamic loading at both high and low temperatures. Thus, there remains a need for a polyurethane elastomeric belt that has excellent load carrying capability as well as the characteristics necessary to withstand dynamic loading under high and low temperature conditions, whether such belts are the form of power transmission belts, V-belts, micro-ribbed belts and the like.